One example of the angular velocity sensor of this kind is shown in FIG. 11. In FIG. 11, oscillating element 4 is provided with drive electrode 1, monitor electrode 2 and detecting electrode 3.
The output side of drive circuit 5, the input side of detecting circuit 6 and the input side of monitor circuit 7 are connected with drive electrode 1, detecting electrode 3 and monitor electrode 2, respectively.
The output side of monitor circuit 7 is connected with the input side of rectifying circuit 8, which outputs a signal rectified from a signal inputted from monitor circuit 7. The output side of rectifying circuit 8 is connected with the input side of smoothing circuit 9, which outputs a smoothed signal smoothed from the signal inputted from rectifying circuit 8. The output side of monitor circuit 7 is also connected with the input side of oscillation control circuit 10, the output side of which is connected with the input side of drive circuit 5. The output signal of smoothing circuit 9 is inputted as a gain control signal to oscillation control circuit 10.
Smoothing circuit 9 is provided with first resistor 16, second capacitor 17 and first reference voltage 14. The output side of rectifying circuit 8 is connected with one terminal of first resistor 16, the other terminal of which is connected with one terminal of second capacitor 17 and the input side of the gain control signal of oscillation control circuit 10. First reference voltage 14 is connected with the other terminal of second capacitor 17.
The output signal of rectifying circuit 8 is smoothed on the basis of smoothing time constant τ1, as expressed by (Equation 1), if first resistor 16 has resistance R1 and if second capacitor 17 has capacity C2:τ1=R1·C2  (Equation 1).
The smoothed output signal is inputted as the gain control signal to oscillation control circuit 10. This oscillation control circuit 10 controls the smoothed output signal into a gain according to the level of the gain control signal, i.e., the amplitude level of oscillating element 4, as generated by monitor electrode 2, and feeds an output signal to drive circuit 5.
Gain Av of oscillation control circuit 10 is expressed by (Equation 2), if the reference voltage of oscillation control circuit 10 is designated by Vref, if the gain control signal or the output signal of smoothing circuit 9 is designated by Vc and if the amplification factor of oscillation control circuit 10 is designated by A:Av=A·(Vref−Vc)  (Equation 2).
In case the transmission impedance of oscillating element 4 is increased by disturbing conditions such as a temperature change, the drive amplitude of oscillating element 4 decreases with respect to the drive signal fed from drive circuit 5 to drive electrode 1. As a result, all of the charge to be generated in monitor electrode 2 in accordance with the oscillation level of oscillating element 4, the output voltage of monitor circuit 7 and the output signal of rectifying circuit 8 decrease so that gain control signal Vc or the output signal of smoothing circuit 9 decreases. On the basis of (Equation 2), gain Av of oscillation control circuit 10 increases, and all the output signal of oscillation control circuit 10, the output signal of drive circuit 5 and the voltage to be applied to drive electrode 1 increase, so that the drive amplitude of oscillating element 4 increases. As a result, oscillating element 4 is controlled to an initial amplitude level so that a stable sensor output can be achieved even under the disturbing conditions such as the temperature change.
The prior art relating to the invention of this application is disclosed, for example, in JP-A-9-281138.
In this configuration of the prior art, however, a resistor and a capacitor with large constants have to be prepared for the smoothing circuit. Especially when the smoothing circuit is configured of an IC (Integrated Circuit), the area for the resistor and the capacitor to occupy is enlarged to harm the size reduction and the circuit integration of the sensor.